This invention is related to the general field of electronic imaging, and more specifically, to methods and apparatus for producing electronic video signals from a photographic original. The invention is particularly applicable to linear array scanning devices employed in telecine machines.
Telecine machines are used to produce television or video images from photographic film originals. Three basic methods have been developed to implement the film to video conversion. The first method uses a television camera arrangement on which the photographic film is projected. The second method employs the use of a flying spot scanner system in which a raster on the face of a special cathode-ray tube is imaged on the film and received by photomultiplier tubes. The third and most generally preferred method employs the use of a solid-state linear array film scanner to scan the photographic film one line at a time.
A linear array film scanner typically employs a light-sensitive linear charge-coupled sensor device (CCD) that provides a serial output representing a single video line. The photographic film to be transferred is transported between the CCD sensor device and a light source and an optical system is used to focus the photographic image present on the film onto the CCD sensor device. The movement of the photographic film provides the vertical (frame) scan rate and the cycling of the CCD sensor device provides the horizontal (line) scan rate.
Generally, three CCD sensors are provided (red, green and blue) if color conversion is to be accomplished and a beam splitter is provided in the optical system to image the illuminated section of the photographic film onto all three CCD sensors at the same time. A three CCD sensor system is described, for example, in U.S. Pat. No. 4,205,337. Alternatively, a single CCD sensor can be employed as illustrated in U.S. Pat. No. 4,736,251, wherein three CCD line sensors are formed on a common substrate.
The photographic film is driven at a constant speed, generally at either 24, 25 or 30 frames per second (fps) depending on how the film was originated and to which video standard the film is being converted, during the actual film-to-video conversion operation. It is common practice, however, to operate the telecine machine in a "shuttle" mode between frame rates from near zero fps to over 300 fps. The shuttle mode is used to locate a particular starting point or selected portions of the motion picture film that the operator wishes to convert to a video image signal.
In a telecine machine that employs a solidstate sensor as described above, the sensor scan rate will necessarily depend on the running speed of the film to be converted. Varying the speed of the film during shuttle mode operation will therefore result in wide variations in integration time for the solidstate sensor causing changes in picture quality as film speed is varied. For example, at slow film speeds and relatively long integration times, the charge accumulated may reach the saturation level of the photosensitive devices employed in the CCD sensor resulting in an overexposed image. In contrast, the charge accumulated at high film speeds may be insufficient thereby resulting in an image which appears underexposed.
Efforts have been made to address the problems associated with variations in integration time experienced with solid-state telecine machines. For example, U.S. Pat. No. 4,630,120 issued to Childs (incorporated herein by reference) describes a system that employs the use of variable-gain amplifiers to compensate for changes to the magnitude of the output signal from the CCD sensors. The disclosed system generates a compensation signal based on the reciprocal of the sensor scanning period. The use of the variable-gain amplifiers alone, however, does not provide sufficient compensation for wide variations in integration time.
For example, if a system were calibrated with that maximum charge capacity equal to 1/2 full well at 30 fps, at 24 fps the sensor would reach a charge capacity of 5/8 full well, and at 15 fps the integration time would be twice that of 30 fps resulting in charge capacity reaching full well. Thus, as the scan rate decreases the integration time must be reduced to prevent saturation. This can be accomplished by reducing the integration time by 1/2 below 15 fps, resulting in the sensor charge being at 1/2 full well and rejecting every other scan line via signal processing. This is described in detail in U.S. Pat. No. 4,630,120 as using "dummy" scan lines.
The above-described solution to variations in integration times provides an optimum signal-to-noise (S/N) ratio at 15 fps, i.e., when the sensor is operating at full well, which occurs during shuttle mode operation and not during film-to-video conversion. Thus, the sensor is being operated at less than full well, and consequently not at the optimum S/N ratio, during the actual conversion process. Accordingly, it would be desirable to provide a method and apparatus that compensates for variations in integration time while optimizing the S/N ratio for the sensor during actual film-to-video conversion operations.